This invention pertains to an optical improvement for detecting absorbance and fluorescence in capillaries, and which is particularly useful for capillary electrophoresis and capillary liquid chromatography.
Generally, the sensitivity of ultraviolet detection which can be achieved by using transverse illumination of capillary tubes is limited because only a small fraction of the incident radiation falling on the tube actually passes through the center of the tube where the sample is located. This problem is illustrated more clearly in FIG. 1, which shows a cross-section of a fused silica capillary tube 101 having a center bore 102. (The typical outside plastic coating on the capillary is generally removed in a small region to allow the incident radiation to traverse the tube. Hence no outside coating is shown in the illustrated cross-section.) Several rays are shown, 103, 105, 107, and 109, which traverse the region of the capillary tube 101. In this example it will be assumed that the capillary tube 101 has an outside diameter of 375 .mu.m, and an inside diameter of 50 .mu.m, and that the incident beam has a diameter D of of 500 .mu.m. As illustrated, only about 20 to 30% of the incident beam actually passes through the center of the tube and is detected by photodiode 111. Such a geometry provides an average path length of only about 0.6 times the inside diameter of the capillary tube, i.e. about 30.0 .mu.m. Further, the detector cell volume, i.e. the volume inside of the capillary tube that is illuminated by the beam, is quite large. Both of these factors contribute deleteriously to the signal to noise ratio of the detection system.
What is needed is a detection scheme that maximizes path length and minimizes detector cell volume.